The invention relates to a sheet feeding device, such as a Large-Capacity Cassette (hereinafter referred to merely as LCC), adapted to store a large quantity of sheets to be fed into a sheet processing apparatus, such as an image forming apparatus, that performs predetermined processing on sheets.
Conventional LCCs are designed for installation beside a sheet processing apparatus and for storing sheets of a size most frequently used in the apparatus. Some LCCs adapted for connection to copiers as image forming apparatus have a capacity of approximately 2,000 sheets of A4-size plain paper placed in landscape orientation.
An LCC has a sheet stacker provided with an internal stacking plate for stacking sheets thereon. The stacking plate is rendered movable up and down within a predetermined range. The sheets on the stacking plate are fed into the apparatus sequentially from top to bottom. As the number of sheets on the stacking plate decreases, the stacking plate is raised. When replenished with sheets, the stacking plate is lowered. With a small number of sheets stacked on the stacking plate, there is a large space formed under the stacking plate inside the sheet stacker. Sheets mistakenly placed in the space prevent the stacking plate from being lowered, which may cause damage to the LCC.
As a solution to the problem, JP H09-086681A discloses an image forming apparatus provided with a sensor for detecting a stacking plate at its lowest level in a movable range. The prior art apparatus measures the time elapsed between the moment when the stacking plate starts to be lowered and the moment when the sensor is turned on. When the measured time is shorter than a reference time, the prior art apparatus determines that sheets or the like are placed in a space under the stacking plate, and sends out a warning.
A recent trend has been an increasing variety of types and sizes of paper for use in image forming apparatus along with an increasing number of paper sheets to be processed at one time. This trend has been driven by more and more uses and functions of recent image forming apparatus.
In accordance with the trend, an LCC has been proposed that has a sheet stacker with a capacity of more than 4,000 sheets of a single size to be chosen from multiple sizes. With such LCC, there is always an open space on a stacking plate inside the sheet stacker except in a situation where sheets of maximum storable size are stored in the sheet stacker.
It is assumed that an object other than sheets for feeding into the image forming apparatus (hereinafter referred to merely as an object), such as replenishment sheets, should not be placed on the stacking plate inside the sheet stacker. However, the open space on the stacking plate allows room for placing such an object. In a case where an object, as well as sheets for feeding, is placed on the stacking plate, the sheets are sequentially fed and decrease in number, until the object becomes higher in height than the rest of the sheets. When the stacking plate is raised, thus, the object is brought into collision with an upper inner surface of the LCC. This collision may cause damage to the LCC.
In order to raise the stacking plate to position a top sheet at a sufficient level for feeding, conventional LCCs have detection means for detecting the level of the top sheet. Such detection means is located near a downstream end, with respect to a sheet feeding direction, of stacked sheets of paper for feeding. However, such means is not adapted to detect presence of an object on the stacking plate. Thus, such means fails to prevent collision of the object with the upper inner surface of the LCC and resultant damage to the LCC.
As a solution to the above-mentioned problem, an LCC has been proposed that is provided with not only a sheet sensor, positioned approximately at a leading end of a top sheet of paper for feeding stacked on a stacking plate, for detecting the top sheet, but also an object sensor, positioned upstream, with respect to the sheet feeding direction, of the sheet sensor, for detecting a top part of an object placed on the stacking plate. Based on the detection result of the sheet and object sensors, the LCC determines whether an object is placed on the stacking plate. More specifically, the LCC determines that an object is placed on the stacking plate, when the object sensor detects the object while the sheet sensor does not detect a top sheet of paper for feeding. Then, the LCC lowers the stacking plate.
However, this configuration involves the use of both of the sheet and object sensors to detect an undesirable object, thereby requiring complicated detection control.
In light of the foregoing, a feature of the invention is to provide a sheet feeding device, with a simple configuration, that detects an object placed on a stacking plate, thereby preventing damage to the device itself.